The quantity of sand used should be such that it completely fills the voids of coarse aggregate. Similarly, the quantity of cement used shown such that it fills the voids of sand, so that a dense mix the minimum voids is obtained.

In actual practice, the quantity of fine aggregate used in the mix is about 10% more than the voids in the coarse aggregate and the quantity of cement is kept as about 15% more than the voids in the fine aggregate.

(4) Maximum Density Method: (Not very Popular)

Where, D = maximum size of aggregate (i.e. coarse aggregate)

P = percentage of material finer than diameter d (by weight)

d = maximum size of fine aggregate.

A box is filled with varying proportions of fine and coarse aggregates. The proportion which gives heaviest weight is then adopted.

(5) Water – Cement Ratio Method:

According to the water – cement ratio law given by Abram as a result of many experiments, the strength of well compacted concrete with good workability is dependent only on the ratio.

The lower water content produces stiff paste having greater binding property and hence the lowering the water-cement ratio within certain limits results in the increased strength.

Similarly, the higher water content increases the workability, but lower the strength of concrete.

The optimum water-cement ratio for the concrete of required compressive strength is decided from graphs and expressions developed from various experiments.

Amount of water less than the optimum water decreases the strength and about 10% less may be insufficient to ensure complete setting of cement. An increase of 10% above the optimum may decrease the strength approximately by 15% while an increase in 50% may decrease the strength to one-half.

According to Abram’s Law water-cement law, lesser the water-cement ratio in a workable mix greater will be the strength.

If water cement ratio is less than 0.4 to 0.5, complete hydration will not be secured.

Does RMC typically have lower strength than a similar grade nominal mix? Theoretically it should as it has a higher water content to ensure better workability. Or is that somehow factored in at Design Stage. Say for instance balancing the fineness of coarse aggregates.